Radio systems are sometimes used in environments where the signals must be protected from detection by others. The signals must be resistant to deliberate blockage and the information being transmitted must be protected from disclosure to others. These capabilities are often referred to as low probability of detection (LPD), jamming resistance (JR) and, low probability of intercept (LPI).
Commercial radio systems, including the broadband wireless communications systems based on OFDM technology such as those of the IEEE standard 802.16, are designed from the outset to operate in protected spectrum channels with low interference and noise. These radio systems are also designed to have signals that are easy to detect in order to minimise receiver costs and maximise performance. While these systems typically provide security for the users' data through traffic encryption, they provide little protection for the internal system control signalling and are readily detected and jammed. Typically the radio signal design includes many features which make them easy to detect, hence they have a high probability of detection by an outside observer.
Fundamentally, the commercial radio systems are designed to operate in dedicated spectrum with no (undue) interference from outside sources. Similarly, the signals are carefully designed with features that make them most easily detectable to receivers, particularly low cost mobiles. The radio transmissions, for example include pilot signals that are intended to act as beacons that are easy for the mobiles to detect, acquire synchronisation and lock to the system channels. Similarly, the timing of transmissions is organised with several regular periodicities that render the signal timing easy to detect and maintain. Such common timing co-ordination helps to ensure high capacity performance for the commercial system. The commercial systems also typically broadcast a paging channel that is used for coordination of the system information and remote station operation. The paging channel is also easy to detect and, if jammed, easily disrupts the operation of the whole system. In order to speed the acquisition by the mobile receivers of the base-station signals, they include prominent and regular features to permit the mobiles to quickly detect them and transfer calls from one station to another (i.e. “handover”). All of these features that are basic to a commercial radio system are opposite to the covertness and resistance to hostile jamming that are required by a military radio system.
FIG. 1 illustrates some of the basic characteristics of a broadband radio waveform. In this illustration time is in the horizontal direction generally increasing towards the right. In the vertical direction is generally shown the sub-carrier, or radio frequency, space of the transmissions. The signal is thus utilising the two dimensions of time and frequency in this illustration. There are other possible dimensions such as spreading code and frequency hopping that may be used but are not shown in this illustration. In the case of commercial transmission systems there are regular intervals for downlink and uplink flow of information. In this case downlink refers to the direction of transmission from the network to the user terminals and uplink refers to the direction from the user terminals to the network. In the case illustrated, Time Division Duplexing (TDD) is shown, in which the direction of transmission in a single transmission channel alternates between downlink and uplink. Radio communications systems may also use Frequency Division Duplexing (FDD) in which the two directions of transmission use channels in different portions of the radio spectrum. The transmissions of the radio system may be continuous, or they may occur only when devices have information to send to each other. In this case, transmission bursts with a format equivalent to that shown in FIG. 1 are sent when there is data or signalling to communicate, and there are no transmissions at other times.
The transmission for the downlink includes a number of elements. These include a preamble portion that may be used by the receivers to acquire the transmission timing, a broadcast control portion that may be used by receivers to learn the organisation, format and timing of the information in the transmission, common broadcast information for multiple users, and transmissions of data directed for individual user terminals. These transmissions may employ a combination of time, spreading code, frequency hopping or sub-carrier groupings for the elements.
The transmissions in the uplink are similarly divided into similar elements including preambles, control information and data from individual user terminals. These transmissions also may employ a combination of time, spreading code, frequency hopping or sub-carrier groupings for the elements.
FIG. 2 illustrates some of the features of a broadband radio waveform in the frequency domain. In this illustration the horizontal axis is the frequency space of the communications channel. The vertical axis shows the strength of signals at the various sub-carrier frequencies. Not shown is the phase of the multiple radio signals. This illustration shows that the communications channel may include modulated traffic sub-carriers that may be used for transmission of user data, pilot sub-carriers that may be used for synchronisation of the radio receivers and guard sub-carriers that may be used to separate the radio signals from other signals in the band. There may also be other sub-carriers in the channel that are introduced, for example, to control the Peak to Average Power Ratio (PAPR) of the radio signal. These sub-carriers typically do not contain any user information, but have a modulation state chosen to reduce the peak power of the composite signal. The illustration also shows that the pilot sub-carriers typically are transmitted with a higher power level and with a fixed modulation pattern that is known to the receivers. These known signals may be used by the receivers to estimate the radio channel and so improve the communications performance. The illustration shows the various sub-carrier signals in locations in the frequency space. In order to facilitate sampling of the radio channel and to provide robustness to interference, the allocation of sub-carriers to various functions changes from symbol to symbol. This variation, however, follows a fixed pattern that is well defined in the radio system standard and so does not prevent the signal feature detection by an adversary receiver.
FIG. 3 illustrates the general apparatus and process arrangement for forming the broadband radio waveform. This system processes user data (this may be Forward Error Coded (FEC)) from the scheduler and other parts of the transmitter system. This user data is gathered together with other pilot, PAPR and guard sub-carriers in a modulation format. This information is processed by an Inverse Fourier Transform (IFFT) process to form a time sequence. For most intervals of transmission, the time sequence symbols are appended with a cyclic prefix and frequency translated and amplified to form the desired radio frequency signal that is transmitted by the antenna. Typically, at the beginning of each transmission frame, preamble symbols (one or more) are introduced to mark the start of the frame. These symbols are designed for easy detection by the receivers using a known correlation pattern. In this illustration the preamble symbols are shown as being injected at the appropriate time through a switch after the IFFT. The preamble symbols may equivalently be introduced as special data patterns input to the IFFT, instead of the user data, at the preamble symbol times.